2000 Chevy Silverado Idle/shift problem

I own a 2000 Chevy silverado with the 4.8 liter engine. 156K miles.

This pickup runs without flaw about 40% of the time. The rest of the time, it will idle up to 1500 rpm when in park. It will also not shift until the engine has reached 2500-2700 rpm. It has less power. This problem usually occurs after its hot, or has been driven 30 mile or so. The problem can sometimes be corrected by letting it sit overnight or cooling off completelty. I have replaced the TPS. Tested the IAC, the MAF sensor. Have replaced the air filter and fuel filter. Ran Injector cleaner through. Have replaced the air intake manifolds. Took it in to the Chev. Dealer. They charged me $90 to tell me the throttle body was "not responding" They wanted $750 to replace it. This is not the solution